Can ancient tree rings help us understand climate change?
To predict the future, it helps to have a solid grasp of the past. But when it comes to meteorology and climate change, scientists don't have much to work with. Reliable records of hurricanes reach back only two decades. Even data on such basics as temperature and rainfall span less than a century for all but a few sites worldwide. That means scientists have little confidence that the data driving their climate models could predict even known, historical outcomes—let alone future trends. Yet, as international concern about climate change and its implications for human civilization intensifies, so does the need to contextualize current weather patterns within the span of history. "The way forward is to complexify what we know about the past and make it more rigorous," says classics professor Sturt Manning. "If you can get the climate models to predict detailed climate history correctly, maybe they can also work going forward."
A professor of archaeology and classics who specializes in the timelines of events in ancient Mediterranean cultures, Manning is determined to enrich the data available for climate modeling by crafting a record of annual rainfall stretching back more than 600 years for the drought-plagued region he studies. "It's a good laboratory case to be able to work with," says the professor, who plans a similar effort to map the effects of the Indian Ocean monsoon cycle in Oman, with funding from Cornell's Einaudi Center for International Studies.
Both projects—and all of Manning's other scholarly efforts—rely on a basic horticultural reality: trees in temperate regions add a single growth ring in each calendar year. Over time, those concentric patterns vary with local temperature and rainfall, but in any given year the trees of a single species growing in the same region tend to put on matching rings, allowing scientists to correlate those in a tree whose age is known with those in such objects as a piece of charcoal, the frame of a painting, or a piece of ancient building timber of the same species. Scientists working in the field—known as dendrochronology—have already assembled timelines extending from the present back to Tudor England, ancient Rome, the rise and fall of the Egyptian dynasties, and into Neolithic history. Says Manning: "There's no other technique that allows you to go back with confidence that far to a specific year."
As the head of Cornell's Wiener Laboratory for Aegean and Near Eastern Dendrochronology, Manning has access to one of the world's largest collections of tree rings—including more than ten million measurements based on samples collected by his predecessor, classicist Peter Kuniholm. "Peter started almost from ground zero—he's been an evangelist," says Manning, who sees himself as part of a second wave in the field, deploying data in new directions to test fresh hypotheses. Among those new techniques are dendro-climatology—the use of tree rings to deduce historical climatic variations—and dendrochemistry, an emerging field that mines the chemical markers embedded in each ring to reveal the soil- and airborne particles a tree incorporated in a given year, offering clues to extreme weather phenomena such as hurricanes and major pollution events like volcanic eruptions.
Manning, a self-described "classic itinerant Australian" who has held appointments at the University of Toronto and the U.K.'s University of Reading, traces his interest in the climatic clues hidden within trees to an early curiosity about the relationship between weather patterns and historic events. Modern-day Athens, he notes, gets barely enough rainfall to support a single crop of wheat. Yet in the eighth century B.C., the city gave rise to a cultural renaissance that generated the precursor to our own alphabet as well as Homer's Iliad and Odyssey. The period was also marked by a drastic reduction in sunspots and solar flares, known as a solar minimum. "You would have had significantly more rainfall in the eastern Mediterranean, and a longer growing season," he notes. "Suddenly it makes sense why a number of Mediterranean and Near-Eastern places take off around about 750 B.C." Similar solar minima correspond with both the start of the Bronze Age in 2900 B.C. and the rise of the Venetian Empire in the fifteenth century A.D. "It's not that climate explains history," says the professor, "but it did seem to me that maybe climate provided a context."
Manning's most recent field research included a visit to Israel, where his team identified an Iron Age oak beam they hope holds clues to the timing of the Biblical King David's reign. In Cyprus, the team has focused on filling in gaps in the known dendro timeline. And closer to home, lab staff are analyzing samples from historic buildings on the Hill and throughout Upstate New York and in the Great Lakes region. "He's incredibly broad in his approach, integrating several different disciplines, scientifically sophisticated approaches to problems," says University of Arizona dendrochronologist Jeffrey Dean. "He's good at defining a problem and then focusing all sorts of different evidence—archaeology, dendro-chronology, ice cores, radiocarbon dating—to answer it."
Kuniholm, who headed Cornell's dendro department for three decades, says that while the theories underlying dendroclimatology hold promise, Manning has his work cut out for him. Scientists still struggle to understand even day-today weather patterns, he notes; discerning the conditions of previous centuries to predict broad future trends will be no easy task. "Forecasting the weather is a stinker," says Kuniholm. "You listen to the radio to find out what the weather will be that day, and half the time it doesn't happen." Even so, he says, Cornell's culture of collaboration could work to his successor's favor. "There are lots of people at Cornell who do climate, climate change, lower atmosphere, upper atmosphere, natural resources. Cornell welcomes this kind of thing."
Even before his January 2006 joint appointment to the departments of Classics and Earth and Atmospheric Sciences, Manning used the tree ring collection to investigate the date of the eruption of Thera. The Minoan volcano's cataclysmic Bronze Age explosion spread archaeological evidence throughout the Mediterranean and sparked massive climate change in the Aegean and beyond, causing crop failures as far away as China. Classicists have used the event as a key marker for assessing relationships among contemporary civilizations including ancient Greece, Egypt, and Cyprus, but estimates of the date itself varied by a century, spurring passionate debates over which cultures preceded, influenced, and dominated others. Manning's findings, published in Science five months after he came to campus, narrowed Thera's explosion to within the first twenty-seven years of the seventeenth century B.C.—100 years earlier than previously believed. "If you move something fifty or 100 years, that starts to mean a whole lot of history has to be rethought and rewritten," says Manning. "It would be a bit like discovering that America fought its war for independence not against the British, but the Germans or French."
— Sharon Tregaskis '95